There is evidence to suggest that exposure of pregnant women to tobacco smoke is related to higher childhood blood pressure in their offspring. It is not well known whether this association is set in utero or by shared postnatal environments. The objective of this study was to assess the association between tobacco smoke exposure of pregnant mothers and blood pressure and heart rate of their newborns. In an unselected birth cohort, blood pressure and heart rate were measured in 456 infants at approximately 2 months of age. Smoking exposure of mothers in pregnancy was obtained by questionnaire. Of 456 mothers whose infants had blood pressure measured, 363 (79.6%) were not exposed to tobacco smoke in pregnancy, 63 (13.8%) did not smoke in pregnancy but were exposed by others, and 30 (6.6%) smoked. Infant offspring of mothers who had smoked during pregnancy had 5.4 mm Hg (95% CI: 1.2 to 9.7; P=0.01) higher systolic blood pressure levels than offspring of mothers who were not exposed to tobacco smoke in pregnancy, taking account of birth weight, infant age, gender, nutrition, and age of mother. No associations were found between maternal exposure to tobacco smoke in pregnancy and diastolic blood pressure. A positive association between maternal exposure to tobacco smoke and heart rate was largely explained by confounding. It can be concluded that maternal exposure to tobacco smoke in pregnancy has a substantial increasing effect on systolic blood pressure in early infancy.